A voltage transformer steps down the voltage of a circuit to a low value that can be effectively and safely used for operation of instruments, such as voltmeters, watt meters, and relays used for various protective purposes. They are designed for relatively low power but high accuracy and high reliability.
A problem lies in the design of an insulation system for a voltage transformer that will withstand the use to which is intended. The most likely contributor to failure of an insulator under electric field stress is partial discharge. Partial discharge, also referred to as corona, is the ionization of a gas, such as air, within a void or gap in the insulation system when the electric field stress exceeds a critical value. The magnitude of the discharge is dependent on the number and size of the voids within the insulation system. The consequence of allowing discharges to occur anywhere within the insulation system is a significant reduction in the life of the insulating material. Organic insulating materials will ultimately fail when exposed to continuous discharge conditions, and the time to failure usually varies as the inverse of the voltage stress.
Medium voltage resin-cast voltage transformers have low and high voltage windings of concentric layer-wound type coils. The transformer coils consist of layers of low and high voltage windings and shields that are disposed so as to give uniform distribution of electrical field, thus reducing partial discharge and increasing impulse voltage strength. A major requirement for the reliable and safe operation of the voltage transformers is the performance of its insulation. Partial discharge activity can initiate under normal working conditions in high voltage equipment in cases where the insulation condition has deteriorated with age and or has been aged prematurely by thermal over-stressing. Thus, the voltage transformer needs to be designed so that the electrical field is below the critical value of the insulation system under normal working conditions. Because of the high non-uniformity of the electrical field, it requires techniques and methods for its regulation below the critical value.
Field regulation techniques may be used in any transformer or inductor, but are more applicable to those encapsulated in resin. One of the techniques used for making the electrical field more uniform is using electrical shielding with rounded electrodes. These shields have traditionally been made of conductive materials, usually folded edge copper foils. However, there are a number of complications and difficulties inherent in making of this kind of shield. For example, it is difficult to fold the foil because it is not possible to make a full turn and it is not possible to achieve a uniform radius throughout the entire shield length. Further, the edges of the foil can create problems with field regulation. The combination of the non-uniformity of radius and edges prevent these foil-based shields from maintaining a uniformity of the electrical field.
A number of issued patents address the issue of avoiding partial discharge activity, but each has significant drawbacks. For example, U.S. Pat. No. 2,942,215, titled “Corona shield” discloses the use of an insulated conductor that is disposed along the edge of the windings between the core and the windings of the transformer and grounded. These shields are said to distribute the dielectric stress that builds up on the edges of metal parts adjacent to transformer winding subjected to high potentials to prevent corona. However, the purpose of these shields is to regulate the electrical field of the edge of the ground electrode magnetic core and, hence, control the field at one point. Accordingly, they do not provide uniform voltage distribution over the entire winding space and do not control the electrical field at the edge of the coil. Further, the fact that these shields are grounded makes them unsuitable for use in medium voltage resin-cast voltage transformers.
U.S. Pat. No. 3,678,428, titled “Interwinding shield for power transformers” discloses a shield constructed from interleaved layers of insulating and conducting strips that are assembled into an insulating member and placed around the low voltage winding of the transformer. The interleaving of this shield makes is very difficult to manufacture, as each separate layer must be precisely manufactured and each layer precisely arranged and secured. Further, the interleaving arrangement is not a robust mechanical arrangement and the precisely manufactured layers are easily bent out of their desired orientation.
U.S. Pat. No. 3,699,488, titled “Distribution Transformer Having Static Shield”, discloses a shield consisting of a strip of aluminum-backed crepe paper overlaid on the outer wire turn of the transformer. The edges of the strip are folded over and flattened so that the aluminum is on the inside of the fold to provide minimum edge corona. This shielding has the same drawbacks as those discussed above; namely they are difficult to fold and the edges create non-uniformity in the field. Further, because these shields have elongated thin strip of insulation, such a paper with conductive coating, they are complex and costly to manufacture.
U.S. Pat. No. 4,379,999, titled “Electrostatic shield for a transformer” discloses an electrostatic shield for an electrical transformer that has a substantially ring-shaped inner insulator of asymmetric vertical cross-section with one surface being substantially planar and the other being curved. This shield is formed from multiple layers of conductive foil, and multiple layers of other insulators, including at least one layer of polyethylene terephthalate (PET) film and at least one mica insulation layer with mica bonded to a non-conductive backing film such as glass tape or PET film by a bonding agent such as epoxy resin. This shield is said to improve the dielectric strength of the electrostatic shield. However, it is difficult to manufacture due to the use of foils and multiple different insulting layers that must be bonded together. Further, the fact that it is an outer shield in which both coils are inside the shield limits its applicability.
U.S. Pat. No. 4,652,846, titled “Small transformer with shield” discloses a transformer having and a stamped metal foil frame as the shielding wall between the adjacent face flanges of the coil. The stamped metal foil frame is a single piece of flat metal foil having non-folded edges and an opening between two of the four sides. This shield is much easier to manufacture than the folded foil and multiple layered shields of other devices. However, it is not effective at preventing partial discharge, which is required in the high voltage applications, due to the fact that the sharp edges of the stamped shield create an unstable field, which cannot be easily controlled.
Finally, U.S. Pat. No. 4,845,453, titled “High-voltage voltage transformer with shields” discloses a high-voltage voltage transformer having a core surrounded coaxially by a high and low voltage windings and a slotted metal shield, a shielding electrode at ground potential and surrounding the high and low voltage windings, and a discharge electrode spaced at a slight distance from the metal shield. However, this shield is an outer shield used in construction of high voltage instrument transformers of head type and is not adapted for use in medium voltage resin cast voltage transformers. Further, this shield is also complex and expensive to manufacture and requires the use of special manufacturing tools.
Therefore, there is a need for an electrostatic shield for medium voltage resin cast voltage transformers that will allow the voltage transformer to withstand the use to which is intended, that will provide a uniform distribution of electrical field, that will reduce partial discharge and increase impulse voltage strength, that is relatively inexpensive and easy to manufacture, that is not easily damaged during the manufacturing process, and that is not limited to use in transformers in which both coils are inside of the shield.